The use of inorganic particles in polymeric materials to improve various properties is well known in the art. The method of dispersing inorganics throughout the polymeric materials is generally accomplished by extrusion and other mechanical high shear processes. When dispersing inorganics in polymers, the goal is to achieve a homogeneous dispersion of extremely small sized particles. In this regard, it has proven difficult to achieve a homogeneous dispersion of inorganic particles when the particles have a particle size of less than 100 nm. When the size of the particles are reduced to the nanometer size range, the surface area of the inorganic particles in the polymer increases by an order of magnitude, thus increasing the interactions between the inorganic particles and the polymer by an order of magnitude. This increased interaction in turn significantly increases viscosity of the polymer making it more difficult to homogeneously disperse the inorganic particles.
Boehmite aluminas, particularly those derived from alkoxide precursors, can be produced in high purity via effective control of crystallite size and provide a source of thermally stable nano-particles of high surface area and controlled porosity. According to U.S. Publication 2005/0239945, incorporated herein by reference for all purposes, alkoxide derived boehmites which have been surface-modified with certain sulfonic acid modifiers produce nano-sized particles dispersible in a media. Published accounts also demonstrate that boehmite alumina can be surface-modified with organic saturated and unsaturated carboxylic acids to produce crystallites having an average size of 200-300 nm.
Acrylic acid has also been used to modify inorganics, particularly to couple calcium carbonate with polypropylene. However, acrylic acid was not effective at dispersing the calcium carbonate in a nano-sized dimension.
It is known in the art that alumina dispersions can be obtained by electric charge through the addition of mineral and organic acids. These types of alumina dispersions can be easily dispersed in aqueous systems and form stable sols due to electrostatic stabilization. This electrostatic stabilization is due to an electric charge on the alumina surface caused by the adsorption of protons which are produced from an acidic dissociation mechanism.
It is also known in the art that alumina can form dispersions in organic systems including organic acids and their salts due to a mechanism of electro-steric stabilization. For example alumina dispersions can be obtained by the addition of organic carboxylic acids and the ensuing deprotonation of the organic acid and adsorption of the carboxylate anion (COO—) through an electrostatic retention mechanism. Electro-steric mechanisms can improve the wetting of the alumina particles and their incorporation into a polymer, but loosely bound adsorbed species can also revert or desorb from the surface, and compete with traces of residual water adsorbed on the surface of alumina. Because of the hydrophilic nature of the alumina surface these effects will reduce the solubility of the alumina particles in polymeric media and thus reduce the ability to disperse the alumina particles. Thus, rather than containing fully dispersed alumina, the polymer will contain subdivided alumina particles in the form of larger agglomerates comprising several alumina crystals, thus reducing the number of interfaces and the performance of the polymer.
It is further known that products different from alumina, for example alumoxane, can be prepared by digestion, i.e., decomposition to small fragments, of pseudoboehmite with a very large excess of a carboxylic acids of small molecular weight, e.g. hexanoic acid (see A. R. Barron, J. Mater. Chem. 5(2) (1995) 331-341). The preparation of such alumoxanes is carried out in organic solvents as they require a total absence of water. Additional process steps are thus required such as an extended filtration process to wash out the excess organic solvent and distillation under vacuum to remove unreacted volatile species.
An object of the present invention is to overcome the drawbacks of the methods indicated above. Another object of the present invention is to provide a process for preparing functionalized surface-modified alumina compositions, which include an aqueous phase. Another object is to obtain a functionalized surface-modified alumina composition that is able to dissolve in organic polymer forming nano dispersions which are characterized by nano size particles in the form of single alumina crystals (measured by means of electron microscopy).
The disadvantages of the prior art are overcome by the present invention and a new method to modify and functionalize the surface of an alumina composition as well as a new method of producing functionalized, surface-modified alumina which is dispersible into nano-sized particles in an organic polymer are hereinafter disclosed.